The invention relates to a thread cutter for a sewing machine with a rotating hook, wherein a surface cam is provided which, together with a cam follower, controls the movement path of a pivotable thread catcher lever, which grasps the thread with a thread catcher and severs it with a cutter.
A thread cutter of the type described above is known from DE 23 38 473 C2. However, with the thread cutter described there, two cams arranged on the main shaft of the sewing machine are provided which actuate the thread catcher lever and a separate thread draw-off finger at different times during the rotation of the hook. A large number of mechanical parts and a considerable requirement for installation space result from this, so that difficulties can arise in the course of housing the trigger and drive of the thread cutter in the front free arm of a machine. The adjustment of the sewing machine is also elaborate, because the rotating hook and the two cams on the main shaft must be separately set in respect to the angle of rotation position after the final assembly of the machine.
A further thread cutter for a post-bed sewing machine is known from U.S. Pt. No. 4,138,958, wherein a pin, which puts the thread cutter into motion in a translatory manner, is moved in the axial direction in a control cam of the hook shaft. The construction is the result of the structural uniqueness of a post-bed sewing machine, so that the thread cutter described in this publication cannot be structurally transferred to the shape customary with household sewing machines.
It is the object of the present invention to provide a thread cutter for a sewing machine, which can be housed in a particularly space-saving manner in the front free arm.
In accordance with the invention, this object is attained in that the surface cam is connected in a torsion-proof manner with the hook and that arresting means are provided which, in their blocking position, maintain the cam follower which is coupled with the thread catcher lever radially outside the engagement range of the surface cam and, with manual or program-controlled triggering, only release the cam follower for contact with the surface cam during one revolution of the hook, during which the needle is outside of the pivot range of the thread catcher.
The main advantage of the thread cutter in accordance with the invention lies in that it consists of only a few mechanical parts, which can be housed without problems in the front free arm of a sewing machine. It is also advantageous that a pre-setting of the unit is possible because of a preferred arrangement of the surface cam on the hook shaft, so that a subsequent adjustment of the thread cutter after the sewing machine has been assembled is no longer required.
Further preferred measures, which aid in a further reduction of the requirement for structural space, are the placement of the cam follower directly on the thread catcher lever and/or the placement of the surface cam axially directly next to the hook, preferably on the back of the hook body.
It is furthermore particularly preferred to seat the thread catcher lever pivotably on the hook frame, because in that case the whole unit of hook and thread cutter can be pre-assembled and set.
In a preferred further embodiment of the invention the arresting means have a movable detent element, which in the blocking position maintains the thread catcher lever, which is pre-biased by a spring in the direction of pivoting-in, in a radial outer position in which the cam and the cam follower are out of engagement, wherein it is particularly preferred to disengage the detent element against the force of a restoring spring with the aid of an electromagnet, so that the cam follower of the thread catcher lever comes to rest against the surface cam because of the force of the spring. Besides the compact construction of such a solution, the simple control of the arresting means with the aid of the electromagnet should be stressed. The spring-loaded restoring spring offers the advantage that an automatic detent, and therefore blocking of the detent element, is possible by means of the appropriate pivoting of the thread catcher lever to the outside, wherein in a further preferred embodiment of the invention the control cam of the surface cam has a short cam section, which has the greatest radius of the control cam and with the aid of which the thread catcher lever can be conveyed into the blocking position in which it is locked together with the detent element.
To prevent a collision of the thread catcher lever it is important, because of the hook which revolves twice per needle lift, to permit pivoting-in of the thread catcher lever only when the needle is not in the pivot range of the thread catcher. For example, this can be achieved already in that the arresting means can only be unblocked at defined times and manual triggering is performed for example with a time delay until the needle has left the engagement range.
It is advantageous to provide an additional mechanical safety in that the arresting means preferably have an additional bolt which arrests the thread catcher lever in the radially outward position during the revolution of the hook shaft during which the needle is in the engagement range of the thread catcher lever. Erroneous triggering is dependably prevented by the additional bolt, wherein preferably the additional bolt is coupled with a cam follower, which acts together with a cam which essentially extends over 180 degree on the main shaft in the base of the sewing machine. Such a mechanical solution makes a pivoting-in movement of the thread catcher lever impossible and offers additional operational dependability in respect to an additional bolt, for example operated electromagnetically.
With a winding shaft which extends vertically in respect to the main shaft, in a further preferred embodiment the cam provided on the main shaft is designed as an axial cam, and the cam follower is coupled with a toothed rack as the additional bolt which is in contact with teeth around a pivot shaft of the thread catcher lever. Thus, only a linear guide with only one movable, possibly multi-part element, for the additional bolt is required, so that the additional mechanical safety can be provided simply and in a space-saving manner. With this solution the return of the additional bolt takes place interlockingly by means of the return movement of the thread catcher lever into its locked position.
A particularly preferred embodiment of the present invention provides that a catch is arranged on the hook shaft, which takes the thread catcher lever along in an interlocked manner over at least a partial distance during the pivoting-in movement when the latter is disengaged. The interlocked take-along movement, which can be provided, for example, by at least one toothed cam on the hook shaft, which acts together with teeth on the thread catcher lever, permits an interlocked taking along in addition to the frictionally connected pivoting-in of the thread catcher lever by means of a spring, so that the actual performance of a cutting process following the triggering of the arresting means is assured. Otherwise it would be possible under unfavorable circumstances, for example if using an unsuitable thread, which is caught between the cutter and the thread catcher and therefore blocks the thread catcher lever, that a delayed, uncontrolled pivoting-in movement of the thread catcher lever occurs, wherein the latter can no longer meet the thread loop, for example.
To prevent the thread, which has already been grasped by the thread catcher lever in the course of the movement of the thread catcher to upper dead center and the subsequent sliding out of the hook path, from jumping out of the eye of the thread catcher lever, it is provided in a preferred further development of the invention that the control cam of the surface cam has a holding area which, prior to the cutting of the thread, moves the thread catcher into a position in which the lateral entry opening of the eye for receiving a thread is covered by a stationary element, for example the cutter.
To obtain a sufficient length of the needle thread for re-sewing after cutting, it is provided in a further preferred embodiment of the invention that a holding edge in the area of the entry opening of the thread eye and a pressure edge at the front end of the thread eye for pressing the thread against the cutter are placed at a defined distance from each other wherein, as a further measure, the thread eye is designed as an elongated hole and is divided by a transverse strip forming the holding edge, which has a longitudinal slit for receiving the thread. By means of this it is possible without problems to achieve the required length of the needle thread of approximately 25 to 30 mm without additional means, such as a thread puller or the like. It is further practical to design the control cam of the surface cam in such a way that the severing of the thread only takes place shortly before upper dead center of the thread catcher lever has been reached.
A further preferred design of the invention relates to the thread tension which prior to the cutting process can preferably be set to zero, for example by triggering the electromagnet for disengaging the thread catcher lever simultaneously with the triggering of a step motor for reducing the thread tension. The reduction of the thread tension prior to severing the thread prevents that, following severing, the thread slides out of the needle eye under its inherent tension and must again be threaded in a cumbersome way.
An exemplary embodiment of the invention will be discussed in detail in what follows by means of the attached drawings.